A steel material discharged from a furnace is sent via a roughing mill to a finishing mill. FIG. 6 is a perspective view showing an example of a layout of a hot rolling mill which performs a hot rolling process following on from a finishing mill 102. As shown in this FIG. 6, the steel material undergoes continuous rolling in a finishing mill 102 provided with a plurality of stands F1a to F7a. Then, a steel sheet 103 hot rolled to a desired thickness via the final stand F7a, is conveyed by a run-out table 104 provided with a plurality of conveying rollers 104a. 
The run-out table 104 is installed inside a cooling apparatus 105 located on the downstream side of the finishing mill 102. Moreover, from a plurality of cooling units 106 provided above this run-out table 104, cooling water is sprayed onto the top surface of the steel sheet 103, thereby cooling the steel sheet 103. The cooled steel sheet 103 is wound onto a coiler 108 via coiler-preceding pinch rolls 107. The coiler-preceding pinch rolls 107 serve the role of guiding the steel sheet 103 to the coiler 108, and the role of maintaining back tension after the trailing end of the steel sheet 103 has passed through the final stand F7a of the finishing mill 102. In FIG. 6, the finishing mill 102 has 7 stands, but in some cases may have 6 stands.
In such a conventional hot rolling mill, the leading end of the high-temperature steel sheet 103 having passed through the final stand F7a of the finishing mill 102, in the interval before reaching the coiler-preceding pinch rolls 107, is fed out in an unpinched state. The steel sheet 103, if tension is not applied to the leading end thereof, may present waviness, and retain the wavy shape while passing through the inside of the cooling apparatus 105. For example, even if tension is applied to the steel sheet 103, the steel sheet 103 may vibrate during threading. If the cooling water is sprayed with the shape of the steel sheet 103 not flat or the steel sheet 103 in a vibrating state, variation in the application of the cooling water occurs in various positions on the steel sheet 103, resulting in temperature differences. For example, if waviness occurs in the shape of the steel sheet 103, surfaces which rise into mounds are formed upon this steel sheet 103. Of these raised surfaces, the cooling water strongly impacts those parts on the upstream side in the conveying direction of the steel sheet 103, but the cooling water impacts those parts on the downstream side in the conveying direction relatively weakly compared to those parts on the upstream side in the conveying direction. This is because the steel sheet 103 moves continuously in one direction.
Conventionally, for the reason described above, because temperature differences occur even when attempting to perform cooling uniformly across the surface of the steel sheet 103, manufacturing a steel sheet 103 having stable quality is difficult.
As described above, in a state in which tension is not applied to the steel sheet 103, waviness may occur in the shape of the steel sheet 103 having passed through the final stand F7a of the finishing mill 102. The trailing end of the steel sheet 103, after passing through each stand F1a to F7a of the finishing mill 102, remains in an unpinched state, without the application of tension, while passing through each subsequent stand F2a to F7a. In this case, in the same manner, the steel sheet 103 may not attain a flat shape, and large waviness may occur. Furthermore, even when tension is applied to the steel sheet 103 for example, the steel sheet 103 sometimes vibrates during threading. In this manner, if the steel sheet 103 is cooled in a state where waviness exist or a state where vibrations occur, variation in the cooling state occurs at various positions on the steel sheet 103, and variation in the quality of the steel sheet 103 increases further.
Incidentally, when the relationship of the boiling heat transfer characteristics resulting from water cooling with respect to the steel material surface temperature and the cooling capacity (heat transfer coefficient) is expressed, the relationship shown in FIG. 7 is obtained. In other words, as shown in FIG. 7, the interval where the steel material surface temperature is from TCH to TM is the transition boiling region, and beyond TM is the film boiling region. Moreover, TCH which indicates the boundary between the nucleate boiling region and the transition boiling region is 400 to 450° C. or thereabouts, and TM is 550 to 600° C. or thereabouts.
In the transition boiling region, if the surface temperature of the steel material decreases slightly, the cooling capacity of the water cooling increases sharply. Accordingly, as shown in FIG. 7, for example, if a position point B has a slightly higher surface temperature in the film boiling region than another position point A of the steel material, and water cooling is performed simultaneously on these two points, the steel material surface temperature changes as shown in FIG. 8. In other words, because the steel material surface temperature at point A is closer to TM than that at point B, point A reaches the transition boiling region immediately and a sharp drop in temperature is seen. On the other hand, because the surface temperature of point B, for a brief period, is within the film boiling region (because point B takes more time to reach TM than point A), the drop in temperature is less drastic, and the transition boiling region is reached later than at point A, after which the temperature begins to drop sharply. Accordingly, for a given period, a large temperature difference occurs between point A and point B.
That is to say, in the case where deviations occur in the application of the cooling water to various positions on the surface of the steel sheet 103 due to waviness in the steel sheet 103 and other factors, then this gives rise to a slight temperature difference between positions on the surface of the steel sheet 103. As a result, between each position, the timing of entering the transition boiling region is slightly different. Due to these slight differences in timing, the temperature differences between the positions widen sharply, and consequently a uniform steel sheet 103 material cannot be obtained. Accordingly, to stabilize the quality of the steel sheet 103, it is important that the steel sheet 103 prior to reaching the transition boiling region, is in a uniform state without waviness, and a state of minimal steel sheet vibration.
In Patent Document 1 below, the applicant proposed a rolling equipment in which one or more sets of pinch rolls are provided on the run-out table.